Spiral or S-curve strain relief for pulling a fiber optic cable

ABSTRACT

A strain relief for distributing strain along an outer surface of an optical fiber cable is disclosed. The strain relief is shaped to surround and tangentially grip the cable via friction between the outer surface of the cable and an inner surface of the strain relief. The strain relief also includes a pulling eye to protect the cable (and a connector) from pulling force (i.e. strain), water and dust. The strain relief is relatively small, low cost and easy to use.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the U.S. patent application Ser. No.12/394,676 (now U.S. Pat. No. 7,672,562), by Jeffrey Fandl and HongboZhang, having the title “OPTICAL CABLE HAVING AN ENLARGED SECTION TOFACILITATE PULLING”, which is being filed concurrently, and incorporatedby reference herein.

TECHNICAL FIELD

The present invention relates to optical fiber cables, such as a cablewith a connector, and more particularly to apparatuses used to installsuch optical fiber cables.

BACKGROUND OF THE INVENTION

When optical fiber cables are ready to be installed, installation toolsare attached to the cables. When a cable is pulled during installationusing current installation tools, substantial strain is applied to thecable. Such strain can result in permanent damage to a fiber(s) insidethe cable, permanent damage to a cable connection, unacceptably highsignal loss and/or expensive repairs and rework. Use of currentinstallation tools can also result in extended installation time,increased cable pulling resistance or obstruction, improper handling ofboth cable and connector, added time and risk of cable damage when apulling eye is disassembled, and/or unacceptable taper adhesive residueon a cable jacket.

Therefore, there is a need for different type of apparatuses, which candistribute the strain effectively and provide ease in the cableinstallation.

SUMMARY OF THE INVENTION

The present invention provides a strain relief, which distributes thestrain effectively and provide ease in an optical fiber cableinstallation by attaching to the optical fiber cable. The strain reliefis shaped to surround and tangentially grip the cable via frictionbetween the outer surface of the cable and an inner surface of thestrain relief.

A method of distributing strain along an optical fiber cable during theapplication of a tensile force to the cable including the step ofattaching an inner surface of a strain relief to an outer surface of thecable, wherein the strain relief is adapted to produce tangential gripto the cable by producing sufficient friction between the outer surfaceof the cable and the inner surface of the strain relief based on a shapeof the strain relief.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 shows three variations of current installation tools for opticalfiber cable installation (FIG. 1 a: a large scale webbed pulling eyewith electrical or friction adhesive tape, FIG. 1 b: a steel cable griptool, FIG. 1 c: “No-tape” type pulling grip);

FIG. 2 is a simplified, perspective view of a spiral strain reliefaccording to an embodiment of the invention;

FIG. 3 shows the spiral strain relief with a cable;

FIG. 4 shows a simplified, perspective view of an S-curve strain reliefaccording to an embodiment of the invention;

FIG. 4A shows blow up of section A in FIG. 4;

FIG. 5 shows the S-curve strain relief with a cable;

FIG. 6 shows one example of fastening the strain relief to the cable,and attaching a dust cap to the strain relief;

FIG. 7 shows the cable with both a stopper and the strain reliefattached;

FIG. 8 shows experimental results of four different embodiments of thepresent invention;

FIGS. 8-1 to 8-4 show four different embodiments of the presentinvention; and

FIG. 9 shows a stopper with serial ID printed on the outer surface ofthe stopper.

DETAILED DESCRIPTION

In the following description, like reference numerals indicate likecomponents to enhance the understanding of the invention through thedescription of the drawings. Also, although specific features,configurations and arrangements are discussed herein below, it should beunderstood that such is done for illustrative purposes only. A personskilled in the relevant art will recognize that other steps,configurations and arrangements are useful without departing from thespirit and scope of the invention.

During installation of an optical fiber cable, the cable is pulledthrough a facility ducting, a conduit or a riser at relatively highspeed. When an optical fiber cable experiences such pulling, substantialstrain is applied to the cable as well. Such strain can result inpermanent damage to a fiber(s) inside the cable, permanent damage to acable connection, unacceptably high signal loss and/or expensive repairsand rework.

Referring now to FIG. 1, shown are three variations of currentinstallation tools for optical fiber cable installation. FIG. 1 a is alarge scale webbed pulling eye 100 utilizing an adhesive tape. A dustcap 101 with a puling eye 103 covers a first end of an optical fibercable. To attach the dust cap 101 to the cable, typically electrical orfriction adhesive tape 102 are applied onto the cable. Since the dustcap 100 needs to be removed after the cable was pulled to desiredlocation during the installation, the adhesive tape 102 have to beremovable. Therefore, the area covered by the adhesive tape 101 tends tobe larger to sustain pulling force because only relatively weak(removable) adhesive tapes can be used for this tool. The nature of thedevice lead to longer time to attach/detach the dust cap 101 andpotential bending when the adhesive tape 102 is attached or removed by atechnician.

FIG. 1 b is a steel cable grip tool. The steel cable grip tool 110 has apulling eye 111 and a woven portion 112. When the woven portion 112 ispushed, it creates a space inside the woven portion 112 and when it ispulled, it reduces the inner space. To install an optical fiber cableusing the tool, the cable is inserted into the inside of the wovenportion 112 by pushing the woven portion 112 (i.e. creating a spaceinside the woven portion 112). Then, when the steel cable grip tool 110is pulled, the woven portion 112 tightened up and holds the cableinside. However, this tool is not suitable for pulling a cable with aconnector since the connector may be damaged by the woven portion 112when the cable was pulled. Also, because constant pulling force isrequired to hold the cables inside of the woven portion, it is notsuitable to use for installing optical fiber cables through congestionducting.

FIG. 1 c is a “No-Tape”-type pulling grip. To install an optical fibercable, the cable is wrapped around the pulling grip 120. However,wrapping a cable creates substantial bending to the optical fiber(s)inside the cable, and if a connector is attached to the cable, it isdifficult to wrap the cable around the pulling grip 120.

Also, by using current installation tools, cable installation can belabor intensive to prepare for cable pulling. When cables are ready toinstall, usually a connector is attached to one end of the cable. Toprotect the connector from dust, strain, and water, often the connector(and a part of the cable) is covered by a device known as a dust cap.The dust cap usually has a pulling eye, which is configured to receiveany suitable means for cable pulling. To attach the dust cap to thecable, the dust cap is typically taped or shrink wrapped to the cable.Therefore, a technician can pull the cable by pulling the pulling eyewith appropriate mean. However, by using the current installation tools,this can result in extended installation time, increased cable pullingresistance or obstruction, improper handling of both cable andconnector, added time and risk of cable damage when the pulling eye isdisassembled, and/or unacceptable taper adhesive residue on a cablejacket.

For example, to prepare the large scale webbed pulling eye 100 in FIG. 1a, the dust cap 101 has to be attached to a cable with an adhesive tape102. Since weak (removable) adhesive tape is used for the adhesive tape102, a relatively large portion of the cable has to be covered by theadhesive tape 102 to prevent the dust cap 101 to be separated from thecable when the cable is pulled by the pulling eye 100. After the cableis pulled to the desired installation location, the adhesive tape 102has to be removed carefully so as not to bend the cable and createbending loss. Since the adhesive tape covers relatively large portion ofthe cable, it is very labor intensive.

Accordingly, it would be desirable to have different type ofapparatuses, which can distribute the strain effectively and provideease in cable installation. One of the apparatuses disclosed in thepresent invention is a strain relief. The strain relief with a dust cap(with a pulling eye) protects a cable (and a connector) from pullingforce (i.e. strain), water and dust. Also, the strain relief should berelatively small and low cost.

Strain relief devices according to the present invention are shaped tosurround and tangentially grip an optical fiber cable via frictionbetween an outer surface of the cable and an inner surface of the strainrelief to distribute strain along the outer surface of the optical fibercable. As embodiments, spiral and S-curve strain relief devices aredescribed in detail.

Referring now to FIG. 2, a simplified, perspective view of a spiralstrain relief according to an embodiment of the invention is shown. Thespiral strain relief 21 has a spiral shape groove 22 from one end of thestrain relief to the other end. The spiral is so spaced that it providesa sufficient contact surface between the spiral strain relief and thecable, and, at the same time, it does not creates a sharp bend in thecable. FIG. 3 shows the strain relief with a cable. In this example, acable 32 is attached to strain relief 21. The spiral strain relief canbe attached at the factory or on-site.

Referring now to FIG. 4, a simplified, perspective view of an S-curvestrain relief according to an embodiment of the invention is shown. TheS-curve strain relief 41 has an S-curve shape groove 42 from one end ofthe strain relief to the other end. The S-curve is shaped to providesufficient contact area with a cable without introducing sharp bends tothe cable (i.e. create only gentle bend if any).

Preferably, at least a portion of the inner surface of a strain reliefcan comprise teeth. The teeth are created such that when a cable isattached to the strain relief, the teeth provide more grip to the cable,therefore increase frictional force between the strain relief and thecable. For example, a portion of the S-curve strain relief 41 can haveteeth 43 as shown in the FIG. 4 (section A) and FIG. 4 a. The teeth 43can be placed all of the inner surface of the strain relief 41 or just aportion of it. Also, the shape of the teeth can be any suitable design.Factors, such as the area and the shape of the teeth within the innersurface of the strain relief, depend primarily on the frictional forcerequired between the cable and the strain relief during the cableinstallation.

FIG. 5 shows strain relief 41 attached to a 3 mm single-fiber cable 52.The S-curve strain relief can be attached at the factory or on-site.

The strain relief of the present invention can be made from any materialas long as it creates sufficient frictional force with the cable. In theembodiments of the present invention, the strain relief devices are madefrom plastic. More specifically, commodity type plastics such as rigidPVC or engineering resins such as Nylon (6, 11 or 12), PBT, and PETincluding 100% regrind resins can be used as a material for the strainrelief.

The strain relief of the present invention can be of any size or shapeas long as it has sufficient frictional force with the cable. Therequired frictional force depends on the size of the cable, a pullingspeed, the outside jacket of the cable, and the material used for thestrain relief. In the embodiments of the present invention, the strainrelief is sized such that the cable with the strain relief can passthrough a ¾ inch diameter conduit.

The strain relief of the present invention can be combined with a dustcap. FIG. 6 shows one example of fastening the strain relief to a cable,and attaching a dust cap to the strain relief. A dust cap 61 is adevice, which makes a connection between a cable and a pulling device.The dust cap 61 usually accompanies a pulling eye 64, which isconfigured to connect to the pulling device (not shown in Figure). Thedust cap 61 with the pulling eye 64 is common in connectors used forMulti Dwelling Unit (MDU) application as disclosed in U.S. patentapplication Ser. No. 12/144,259, filed Jun. 23, 2008, having the title“CONNECTOR-COVER FOR PULLING AND WATER RESISTANCE”. As mentioned above,the dust cap 61 is typically taped or shrink wrapped to the cable, andthis allows the technician to grip the cable for pulling. The dust cap61 with the pulling eye 64 can be any material, having an “eye” in oneend for connecting to any pulling device during cable installation. Asan example, braided expandable mesh tubing material is used inembodiments of the present invention. The other end of the pulling eyeis typically attached to a connector or the cable. However, in theembodiments of the present invention, as it is shown in FIG. 6, the dustcap 61 is attached to the strain relief 62 by fastening with twotie-wraps 63. By attaching the dust cap 61 to the strain relief 62instead of directly to the cable, strain management improvessignificantly, and therefore, the cable can sustain a larger pullingforce during cable installation. Alternatively, a wire or sheet metalstrain relief retainer may be used for fastening. Also, it will beapparent to those skilled in the art that other methods of connectingthe pulling eye with the strain relief can be employed without departingfrom the spirit and scope of the invention.

The cable attached to the strain relief can be any type of cable. Thecable can be single-fiber or multi-fiber cables. The cable can be invarious shape or size. The cable can also include one or more fiberribbons, which have one or more optical fibers joined together with abuffer coating in a linear array.

The strain relief of the present invention can be attached to any typeof cable easily. To attach the strain relief onto the cable, the cableonly needs to be pushed into the groove of the strain relief. However,when the strain relief is attached to the cable, the cable should not bebent to cause permanent damage to the fiber in the cable. As mentionedabove, if the cable is bent after it is attached to the strain relief,the strain relief is designed such that the bend created by the strainrelief is much larger than the cable's bending diameter (i.e. a gentlecurve).

Preferably, the strain relief is attached to the cable far enough from aconnector (if any) to prevent any strain being applied to the connector,but close enough to the connector so that the connector does not becometangled with the dust cap and damage the connector and/or a fiber(s)inside the cable during the cable installation.

The present invention can be used to cables with any connectors, such asMPO, MTP, LC and SC connectors.

The contact area between the inner surface of the strain relief (e.g.spiral and S-curve) and the outer surface of the cable is large enoughsuch that the contact area between the strain relief and the cablecreates sufficient frictional force between them, so that the strainrelief does not slip on the cable when the cable and/or the strainrelief is pulled. To create a sufficient frictional force, the strainrelief maybe tie-wrapped. Since the strain relief attaches to the cablemore tightly by fastening, the tangential force between the cable andstrain relief increases. By increasing the tangential force, thefrictional force between them increases(f_(max static)=μ_(static)F_(normal), where f_(max static) is themaximum static frictional force, μ_(static) is the static frictioncoefficient, and F_(normal) is the normal (tangential) force),therefore, a smaller strain relief can be used to perform the samefunction. For example, a plastic retainer, a wire or sheet metal strainrelief retainer may be used to fasten the strain relief.

Also, during the manufacturing process, an optical fiber cable sometimeshas a large fluctuation from a designed diameter. Because of themanufacturing irregularity, strain reliefs designed for a specific cabletype may not provide sufficient frictional force between the strainrelief and the cable, or may not be able to attach to the cable at all.Fastening the strain relief onto the cable (by tie-wrapping for example)is helpful; however, depending on the material used for the strainrelief, the degree of fastening can be limited and may not providesufficient frictional force between the strain relief and the cableduring the cable installation.

To overcome this issue, a stopper can be attached preceding and adjacentto the strain relief in the pulling direction. By attaching the stopperin the front of the strain relief along the pulling direction, even ifthe strain relief does not provide enough frictional force and thereforethe strain relief starts to slip on the cable to the pulling direction,the stopper prevents the strain relief from further slipping toward thepulling direction. FIG. 7 shows one way to attach a stopper 73 next tothe strain relief 72 on the cable 71.

FIG. 8 shows experimental results of four different configurations ofthe strain relief. In the experiment, a strain relief is attached to a 3mm single-fiber cable and with four different configurations to measurepulling force of each configuration. To measure pulling force, a pullingeye is attached to the strain relief and pulling force is continued tobe applied to the strain relief device until it starts to slip. The fourconfigurations tested in the experiments are 1) no tie-wrap assembly: aS-curved strain relief 81 is attached to the cable 82 (FIG. 8-1); 2)three tie-wrap assembly: 3 tie-wraps 83 is attached to 1) assembly (FIG.8-2); 3) three tie-wrap assembly with a removable tape 84: a removabletape is added to 2) assembly as a stopper (FIG. 8-3); and 4) no tie-wrapassembly with heat shrink tube: a heat shrink tube 85 is added to 1)assembly as a stopper (FIG. 8-4). As shown in the experimental resultsin FIG. 8, the configurations 3) and 4), which use the stoppers cansustain from significantly more pulling force compare, to theconfiguration 1 (i.e. approximately double the pulling force compare tothe configuration 1).

The combination of the strain relief and the stopper sustain fromrelatively large pulling force compared to the strain relief alone,however, the stopper can be used independently. The stopper can betightly attached to the outer surface of the jacket of the cable andconfigured to stop an installation tool from sliding over the stopperwhen the cable is being pulled by the installation tool. As theinstallation tools, for example, a strain relief and/or a dust cap witha pulling eye can be used.

The stopper tightly attaches to the outer surface of the jacket withsufficient frictional force that it remains immobile relative to thecable when the installation tool pushes the stopper during the cableinstallation. The stopper needs to sustain from the pulling force duringthe cable installation, however; it can be kept onto the cable orremoved after the cable installation.

Any suitable material can be used for the stopper as long as theysustain from the pulling force during the cable installation. Forexample, a tape or a heat-shrink tube can be used as a stopper. Afterthe cable installation, the stopper can be removed. However, suitablematerial has to be selected such that when the stopper is removed, itdoes not damage the cable and optical fiber(s) inside the cable, such asby bending. Preferably, the stopper stays on the cable after the cableinstallation. A stopper with relatively strong adhesive is used so thatthe area where the stopper covers on the cable can be shortened.Optionally, the outer surface of the stopper can be printed with one ormore letters. For example, as shown in FIG. 9, the outer surface of thestopper 91 can be printed with serial ID.

The thickness of the stopper is such that it stops the installation toolused during cable installation from sliding over the stopper.Preferably, the thickness of the stopper is at least 0.005 inches. Also,the cross section of the stopper can be any shape or form as long as thestopper stops the installation tool. Preferably, the cross-sectionalshape of the stopper is the same shape as the jacket of the cable suchthat the pulling force during the cable installation distribute evenlyonto the cross section of the stopper.

The cable attaching the stopper can be any type of cable. The cable canbe include one or more optical fibers. The cable can be in various shapeor size. The cable can also include fiber ribbons, which have one ormore optical fibers and each of the optical fibers is jointed togetherwith a buffer coating in a linear array.

Often when cables are ready to be installed, a connector is placed onone end of the optical fiber cable at the factory. When a connector isattached to a cable to be installed, the stopper is placed relative tothe connector such that the optical fiber in the cable those notexperience significant bending during the cable installation.

The stopper can be attached to the cable in various ways. However, if aconnector is going to be attached on the cable, then the material usedas a stopper may need to be attached to the cable in advance. Forexample, if a heat-shrink tube is used as a stopper, it needs to beattached to the cable before the connector is attached.

It will be apparent to those skilled in the art that many changes andsubstitutions can be made to the embodiments of the invention hereindescribed without departing from the spirit and scope of the inventionas defined by the appended claims and their full scope of equivalents.

1. An apparatus for distributing strain along an outer surface of anoptical fiber cable comprising: a single-piece strain relief having anelongated slot along its entire length for receiving the cable and beingshaped to surround and tangentially grip the cable via friction betweenthe outer surface of the cable and an inner surface of the strainrelief, wherein the shape of the strain relief is an S-curve, andwherein the strain relief attaches to a pulling eye and via a dust cap.2. The apparatus of claim 1, wherein the strain relief is made ofplastic.
 3. The apparatus of claim 1, wherein the strain relief is sosized such that the cable with the strain relief can pass through a ¾inch diameter conduit.
 4. The apparatus of claim 1, wherein the opticalfiber cable is a single fiber cable or a multi-fiber cable.
 5. Theapparatus of claim 1, wherein frictional contact area between the innersurface of the strain relief and the outer surface of the cable is largeenough such that the strain relief does not slip with respect to thecable when the cable or the strain relief is pulled.
 6. The apparatus ofclaim 1, wherein at least a portion of the inner surface of the strainrelief include teeth.
 7. The apparatus of claim 1, wherein the strainrelief further comprises a stopper to prevent the strain relief to slipalong the cable when the strain relief is pulled.
 8. The apparatus ofclaim 1, wherein the optical fiber further includes with a connector. 9.A method of distributing strain along an optical fiber cable during theapplication of a tensile force to the cable, comprising the step of:attaching the inner surface of a the strain relief of claim 1 to anouter surface of the cable along the entire length of the strain relief,wherein the strain relief is adapted to produce tangential grip to thecable by producing sufficient friction between the outer surface of thecable and the inner surface of the strain relief based on the shape ofthe strain relief, and the shape of the strain relief is an S-curve. 10.The method of claim 9, wherein a contact surface between the innersurface of the strain relief and the outer surface of the optical fibercable is secured by fastening the strain relief to the optical fibercable.
 11. The method of claim 10, wherein the strain relief is fastenedto the cable by tie wrap.
 12. The method of claim 9 further comprisingthe step of attaching a stopper to the cable so that the strain reliefdoes not slip against the cable when the strain relief is pulled. 13.The method of claim 9, wherein the strain relief is attached to theoptical fiber cable such that no strain is applied to a connectorattached to the cable.
 14. The apparatus of claim 1, whereinsubstantially all of the strain from a pulling device is transmitted tothe strain relief, and the strain relief distributes the strain to thecable via frictional contact between the inner surface of the strainrelief and the outer surface of the cable.
 15. The apparatus of claim 1,wherein the dust cap is attached to the strain relief via a tie wrap.